Suppose there was an iron city, a league long, a league wide, and a league high, full of mustard seeds pressed into balls (Ven. Sujato translation). SN15.6
Here the translation of puṇṇaṃ sāsapānaṃ guḷikābaddhaṃ is,
Therefore the city (inside the cubic league) is completely filled with mustard seeds(?) is called guḷikābaddhaṃ (cuṇṇikābaddhaṃ).
The sinhala translations of Buddha Jayanthi Thripitaka and A. P. Soyza says the city is filled with seeds until it forms a pinnacle of seeds.
Like this;
Culinary techniques can travel and last a long time. While I found no suggestions in a rather superficial search on “mustard balls” of regional contemporaneous use, it might actually refer to an ancient technique because mustard balls (made from paste with horseradish, vinegar, honey or mashed raisins) actually are preserved and last longer than mustard seeds.
Perhaps not. I have never actually met a mustard ball, so I don’t know if seeds are utterly eradicated in the process. Your reasoning sounds reasonable.